Trouble with strptime() conversion of duration time string

Question

I have some duration type data (lap times) as pl.Utf8 that fails to convert using strptime, whereas regular datetimes work as expected.

Minutes (before :) and Seconds (before .) are always padded to two digits, Milliseconds are always 3 digits.

Lap times are always < 2 min.

df = pl.DataFrame({
    "lap_time": ["01:14.007", "00:53.040", "01:00.123"]
})

df = df.with_columns(
    [
        # pl.col('release_date').str.strptime(pl.Date, fmt="%B %d, %Y"), # works
        pl.col('lap_time').str.strptime(pl.Time, fmt="%M:%S.%3f").cast(pl.Duration), # fails
    ]
)

So I used the chrono format specifier definitions from https://docs.rs/chrono/latest/chrono/format/strftime/index.html which are used as per the polars docs of strptime

the second conversion (for lap_time) always fails, no matter whether I use .%f, .%3f, %.3f. Apparently, strptime doesn't allow creating a pl.Duration directly, so I tried with pl.Time but it fails with error:

ComputeError: strict conversion to dates failed, maybe set strict=False

but setting strict=False yields all null values for the whole Series.

Am I missing something or this some weird behavior on chrono's or python-polars part?

Answer 1

General case

In case you have duration that may exceed 24 hours, you can extract data (minutes, seconds and so on) from string using regex pattern. For example:

df = pl.DataFrame({
    "time": ["+01:14.007", "100:20.000", "-05:00.000"]
})

df.with_columns(
    pl.col("time").str.extract_all(r"([+-]?\d+)")
    #                                /
    #                 you will get array of length 3
    #                 ["min", "sec", "ms"]
).with_columns(
    pl.duration(
        minutes=pl.col("time").arr.get(0),
        seconds=pl.col("time").arr.get(1),
        milliseconds=pl.col("time").arr.get(2)
    ).alias("time")
)

┌──────────────┐
│ time         │
│ ---          │
│ duration[ns] │
╞══════════════╡
│ 1m 14s 7ms   │
│ 1h 40m 20s   │
│ -5m          │
└──────────────┘

---

About pl.Time

To convert data to pl.Time, you need to specify hours as well. When you add 00 hours to your time, code will work:

df = pl.DataFrame({"str_time": ["01:14.007", "01:18.880"]})

df.with_columns(
    duration = (pl.lit("00:") + pl.col("str_time"))\
        .str.strptime(pl.Time, fmt="%T%.3f")\
        .cast(pl.Duration)
)

┌───────────┬──────────────┐
│ str_time  ┆ duration     │
│ ---       ┆ ---          │
│ str       ┆ duration[μs] │
╞═══════════╪══════════════╡
│ 01:14.007 ┆ 1m 14s 7ms   │
│ 01:18.880 ┆ 1m 18s 880ms │
└───────────┴──────────────┘